I like to release my app 20-25 days before the big day. So for a Christmas app I would release it on Dec 1st, The reason I do this is because on android they have top new free sections for each category. You stay in the top new free section for 30 days. As you get more and more downloads you move higher and higher up the list. So come Christmas time your app should be close to the top of the category and when users go looking for apps by category yours is close to the top. I found that the potential for making money is higher on iOS but when it comes to marketing android is by far easier to get your app to the top which in turn makes a lot more then iOS.